Electrochemical CO₂ Reduction by Ni-containing Iron Sulfides: How Is CO₂ Electrochemically Reduced at Bisulfide-Bearing Deep-sea Hydrothermal Precipitates?

The discovery of deep-sea hydrothermal vents on the late 1970's has led to many hypotheses concerning chemical evolution in the prebiotic ocean and the early evolution of energy metabolism in ancient Earth. Such studies stand on the quest for the bioenergetic evolution to utilize reducing chemicals such as H₂ for CO₂ reduction and carbon assimilation. In addition to the direct reaction of H₂ and CO₂, the electrical current passing across a bisulfide-bearing chimney structure has pointed to the possible electrocatalytic CO₂ reduction at the cold ocean-vent interface (R. Nakamura, et al. Angew. Chem. Int. Ed. 2010, 49, 7692 − 7694). To confirm the validity of this hypothesis, here, we examined the energetics of electrocatalytic CO₂ reduction by iron sulfide (FeS) deposits at slightly acidic pH. Although FeS deposits inefficiently reduced CO₂, the efficiency of the reaction was substantially improved by the substitution of Fe with Ni to form FeNi₂S₄ (violarite), of which surface was further modified with amine compounds. The potential-dependent activity of CO₂ reduction demonstrated that CO₂ reduction by H₂ in hydrothermal fluids was involved in a strong endergonic electron transfer reaction, suggesting that a naturally occurring proton-motive force (PMF) as high as 200 mV would be established across the hydrothermal vent chimney wall. However, in the chimney structures, H₂ generation competes with CO₂ reduction for electrical current, resulting in rapid consumption of the PMF. Therefore, to maintain the PMF and the electrosynthesis of organic compounds in hydrothermal vent mineral deposits, we propose a homeostatic pH regulation mechanism of FeS deposits, in which elemental hydrogen stored in the hydrothermal mineral deposits is used to balance the consumption of the electrochemical gradient by H₂ generation